1. Field of the Invention
The present invention relates to a ring laser gyro that detects an angular velocity of a rotary motion by using a semiconductor ring laser, and more particularly to a ring laser gyro that is capable of detecting a direction of rotation.
2. Related Background Art
Up to now, as a gyro that detects the rotation, that is, the angular velocity of an object, there have been known a mechanical gyro having a rotor and a vibrator, and an optical gyro. Because the optical gyro enables instantaneous activation and is wide in dynamic range, it is increasingly innovative in the gyro field. The optical gyro is classified into a ring laser gyro, an optical fiber gyro, a passive ring resonator gyro, and the like. The ring laser gyro using a gas laser has already been put into practical use, such as in an aircraft. Also, as the ring laser gyro which is small in size and high in precision, there has been proposed a gyro formed of a ring resonator semiconductor laser on a semiconductor substrate, which is disclosed in Japanese Patent Publication No. 62-39836, Japanese Patent Application Laid-open No. 4-174317, and Japanese Patent Publication No. 6-38529.
The gyro formed of the ring resonator semiconductor laser has features that it can reduce the element size, lessen the power consumption, and shorten a start time as compared with the mechanical gyro having the vibrator. Therefore, the gyro of this type is properly used for a vibration absorption control device that prevents a photographing error due to hand vibrations in a still camera or a video camera.
A beat frequency detected from the gyro has information on an angular velocity. In order to detect the beat frequency, there are a method of converting the vibration frequency of a beat signal into a voltage signal by a frequency-to-voltage converting circuit, a method of directly detecting the beat frequency by a frequency counter, and other methods.
However, the conventional gyro formed of the ring resonator semiconductor laser has not been so designed as to detect the beat signal in a short period of time.
According to one aspect of the present invention, there is provided a ring laser gyro, comprising a plurality of ring resonator semiconductor lasers optically independent of each other, said plurality of ring resonator semiconductor lasers including at least one pair, wherein, for the at least one pair, changes in beat frequency increases and decreases are opposite to each other with respect to one directional rotation.
According to another aspect of the present invention, there is provided a ring laser gyro that changes a cycle of impedance fluctuation between terminals in accordance with an applied angular velocity, has a plurality of ring resonator semiconductor lasers optically independent of each other on a common plane and has a plurality of electric terminals that detect the impedance fluctuation, said ring laser gyro comprising: a first ring resonator semiconductor laser that shortens a cycle of the impedance fluctuation as an angular velocity in a certain direction increases; and a second ring resonator semiconductor laser that lengthens the cycle of the impedance fluctuation as the angular velocity increases.
According to the above structures, since the ring resonator semiconductor lasers are optically independent of each other, even in the case where the gyro rotates in any direction, the cycle of the impedance fluctuation from any ring resonator semiconductor laser is shortened. Also, the impedance fluctuations in both of the ring resonator semiconductor lasers are taken in a signal processing circuit as a cyclic fluctuation of the terminal voltage, and the angular velocity is detected on the basis of a signal whose cycle is reduced, to thereby conduct signal processing on the basis of a signal shorter in cycle than the beat frequency in a stationary state. Also, the direction of rotation is found by detecting and comparing whether the cycle of the signal is reduced in any ring resonator semiconductor laser as compared with that in the stationary state, or not.
Further, according to still another aspect of the present invention, there is provided a ring laser gyro that includes a tapered portion in a part of an optical waveguide, a plurality of ring resonator semiconductor lasers optically independent of each other on a common plane, and a plurality of electric terminals for detecting a change in the impedances of said ring resonator semiconductor lasers, wherein said tapered portion includes a first portion where a width of the optical waveguide is gradually widened along a direction of propagating a laser beam clockwise and a second portion where the width of the optical waveguide is gradually narrowed along the direction, and further comprising a first ring resonator semiconductor laser in which said first portion is longer than said second portion, and a second ring resonator semiconductor laser in which said second portion is longer than said first portion.
The tapered shape of the ring resonator semiconductor laser makes the oscillation frequency of the laser mode in the stationary state different between a clockwise mode and a counterclockwise mode. That is, the waveguide light is propagated while repeating total reflection and in the tapered portion, since an incident angle of the total reflection changes, a difference occurs in the loss with the result that the loss of the resonator depends on an orbiting direction. Because a difference occurs in the loss of the resonator due to the orbiting direction of the laser beam, a difference occurs in the oscillation threshold value of the ring laser, and in a state where two modes exist together and the laser oscillates, a difference occurs in the density of photons of those two modes. Since the density of photons is different, a difference occurs in the oscillation frequency of the laser mode. In addition, in the above tapered shape, the lengths of the first portion and the second portion are reversed between the first and second ring resonator semiconductor lasers. With this structure, the sizes of the oscillation frequencies of the clockwise laser mode and the counterclockwise laser mode are reversed between the first and second ring resonator semiconductor lasers. Thus, an increase/decrease in the cycle of the impedance fluctuation when the angular velocity increases in a certain direction is reversed between the first and second ring resonator semiconductor lasers. Since the ring resonator semiconductor lasers are optically independent of each other, even in the case where the optical gyro is rotated in any direction, the cycle of the impedance fluctuation from any one of the ring resonator semiconductor lasers is shortened. The impedance fluctuation in both of the ring resonator semiconductor lasers is taken in the signal processing circuit as a cyclic fluctuation of the terminal voltage, and the angular velocity is detected on the basis of a signal whose cycle is reduced, to conduct signal processing on the basis of a signal shorter in cycle than that of the beat frequency in the stationary state, thereby making it possible to detect the angular velocity. Also, the direction of rotation is found by detecting and comparing whether the cycle of the signal is reduced in any ring resonator semiconductor laser as compared with that in the stationary state, or not.
According to yet still another object of the present invention, there is provided a ring laser gyro further comprising an absorber or a light shield body between the ring resonator semiconductor lasers, thereby making it possible to prevent optical coupling of an adjacent element that prevents the operation of the ring resonator semiconductor laser with the semiconductor laser.
According to yet still another object of the present invention, there is provided a ring laser gyro in which the shapes of the ring resonators of two ring resonator semiconductor lasers have a mirror image relationship. In those two ring resonator semiconductor lasers, the peripheral lengths and the surrounding areas of the ring resonators are equal to each other, and the shapes of the tapered portion have a mirror image relation. Therefore, a frequency difference between the laser beam clockwise in the stationary state and the counterclockwise laser beam are equal to each other in absolute value, but signs are reversed. In other words, in those two ring resonator lasers, the beat frequencies in the stationary state are substantially equal to each other under the same drive condition.
Since the peripheral lengths and the surrounding areas of the ring resonators are equal to each other, the fluctuations of the beat frequencies with respect to the angular velocity in the two ring resonator semiconductor lasers are equal to each other in absolute value and reversed in sign. Therefore, in the case where the driving conditions of those two elements are identical, the direction of rotation can be detected only by comparing the signals from the above two ring resonator semiconductor lasers with each other.
According to yet still another aspect of the present invention, there is provided a method of processing a signal in a ring laser gyro, in which the cycles of the impedance fluctuations between the terminals of the plurality of ring resonator semiconductor lasers are compared with each other, and an angular velocity and a direction of rotation are obtained on the basis of a signal from an element whose cycle is shorter.
Alternatively, the cycles of the impedance fluctuations between the terminals of the plurality of ring resonator semiconductor lasers are compared with the periods of the impedance fluctuations in the stationary state, respectively, and the angular velocity and the direction of rotation are obtained on the basis of the signal from the element whose cycle is reduced.
According to this structure, the cycles of the signals that represent the impedance changes between the plurality of ring resonator semiconductor lasers are detected by the signal processing circuit, respectively, and the cycle per se of the fluctuation or a difference to the fluctuation period and the stationary time is obtained.
The element whose cycle is shorter or whose period is reduced indicates the direction of rotation of the gyro, and the absolute value of the angular velocity can be detected on the basis of that signal. In this situation, the band of the signal processing can be set to only a high-frequency side from the period of the beat in the stationary state. In this case, a signal from an element whose cycle is longer is out of the signal processing band, but if the fact that the cycle becomes longer is found, there is no problem in an accuracy in the detection and a final output is not adversely affected. Also, since the beat frequency from the gyro is higher than the beat frequency in the stationary state, a response time of the gyro can be lengthened up to the inverse of the beat frequency in the stationary state.